The base stations used by the providers of current day multiple channel wireless communication services, such as cellular mobile telephone and Personal Communication Systems (PCS), typically designate equipment for each single radio channel. This is probably a result of the fact that each base station is configured to provide communication capability for only a limited predetermined number of channels in the overall frequency spectrum that is available to the service provider. For example, a base station may service a pre-selected number of channels, such as 48, of the total number of the channels available to the service provider using multiple sets of receiver and transmitter signal processing components.
However, certain wireless service providers would prefer to employ equipment that would be more flexible, both in terms of where it can be located, as well as in the extent of the available bandwidth coverage provided by a particular transceiver site. This is particularly true in rural areas where cellular coverage may be concentrated along a highway, and for which the limited capacity of a conventional 48 channel transceiver may be inadequate. This may also be true in other instances, where relatively large, secure, and protective structures for multiple racks of equipment are not necessarily available or cost effective, such as for PCS applications.
One way to resolve this difficulty is to implement the base station with a broadband transceiver system making use of a high speed analog-to-digital (A/D) converter and efficient digital filtering algorithms such as the Fast Fourier Transform (FFT) to separate the incoming signal energy into the desired channels. A bit parallel Time Division Multiple (TDM) bus is then used to distribute samples generated by the channelizer to a set of Digital Signal Processors (DSPs), with these typically being one DSP associated with each receive channel. DSPs which demodulate the channel signals to provide voice signals. On the transmit side, this implementation includes an inverse FFT based combiner which receives modulated base band samples from the TDM parallel bus and outputs a combined signal representative of the contents of the communication channel signals processed thereby. For a more detailed description of such a system, please refer to co-pending United States patent application entitled "Wideband Wireless Base station Making Use of Time Division Multiple Access Bus to Effect Switchable Connections to Modulator/Demodulator Resources", Ser. No. 08/251,914, filed Jun. 1, 1994 and which is assigned to AirNet Communications Corp., the assignee of this application.
This broadband transceiver system is thus capable of processing many of the available channels in parallel rather than only a small subset of the available channels. However, certain difficulties exist with such an architecture. In particular, it requires a significant amount of data samples to be processed by the DSPs in real time. There must therefore be an efficient mechanism to route control information among the DSPs to handle events such as handing off the connection from one cell site to another, routing calls to the correct mobile telephone, and correct billing of usage charges. These and other call processing events must be handled in real time so that the cellular telephone user does not perceive any delays in the audio signal.
In addition, because this broadband transceiver system architecture is digital, it is capable of handling different radio protocols at the same time. Specifically, the digital channelizer and combiner can be programmed to handle different protocols for different active channels. Certain protocols may even require coordination between more than one DSP to handle a single channel, such as may be the case for Global System for Mobile Communications (GSM), Time Division Multiple Access (TDMA) and other digital protocols which provide more than one voice channel for each radio channel. Both of these criteria require official communication of control messages only between certain DSP's in the array. These events must also occur in real time so as not to adversely affect the available processing bandwidth of each DSP.
It would also be advantageous to have a graceful degradation capability in such a system whereby if one or more of the DSPs are unavailable, the ability of the base station to handle calls is not adversely affected.